📖 In-Depth Theory
Extracting Aluminium by Electrolysis
Aluminium is the most abundant metal in the Earth's crust — but it is very reactive, making it difficult to extract.
Aluminium cannot be extracted by carbon reduction (it is above carbon in the reactivity series).
Instead, electrolysis of MOLTEN ALUMINIUM OXIDE (Al₂O₃) is used.
CHALLENGE: aluminium oxide melts at about 2050°C — impractically high.
SOLUTION: dissolve aluminium oxide in MOLTEN CRYOLITE (Na₃AlF₆).
Cryolite lowers the operating temperature to about 950°C.
Cryolite does not react and is recovered — it just acts as a solvent.
ELECTROLYSIS PROCESS (Hall-Héroult process):
Molten Al₂O₃ in cryolite is placed in a large steel tank lined with graphite (acts as the cathode).
Graphite anodes dip into the melt from above.
At CATHODE (graphite lining):
Al³⁺ + 3e⁻ → Al (aluminium ions reduced to molten aluminium)
Molten aluminium sinks to the bottom and is tapped off.
At ANODE (graphite rods):
2O²⁻ → O₂ + 4e⁻ (oxide ions oxidised to oxygen gas)
PROBLEM WITH CARBON ANODES:
The oxygen produced at the anode reacts with the hot graphite anodes:
C + O₂ → CO₂
Anodes burn away and must be replaced regularly — an ongoing operational cost.
Copper Purification by Electrolysis
CRUDE COPPER from smelting contains impurities (zinc, iron, silver, gold) — too impure for electrical wiring.
Electrolysis purifies it to 99.99% pure copper.
SETUP:
ELECTROLYTE: copper sulfate solution (CuSO₄(aq)).
ANODE: impure copper — REACTIVE anode (dissolves).
CATHODE: pure copper or stainless steel — grows as copper deposits.
AT THE ANODE (impure copper dissolves):
Cu → Cu²⁺ + 2e⁻ (copper atoms from impure anode dissolve into solution)
Less reactive impurities (gold, silver, platinum) do NOT dissolve — they fall as ANODE SLUDGE at the bottom.
AT THE CATHODE (pure copper deposits):
Cu²⁺ + 2e⁻ → Cu (copper ions from solution deposit as pure copper)
RESULT:
The impure anode gradually dissolves.
Pure copper builds up on the cathode.
The concentration of Cu²⁺ in the CuSO₄ solution stays approximately constant.
The anode sludge is collected — it may contain valuable metals (gold, silver) that offset costs.
Electroplating
ELECTROPLATING uses electrolysis to coat an object with a thin layer of metal.
Applications:
Decorative: gold or silver plating of jewellery, trophies.
Protective: chromium plating of car parts, zinc plating of steel (galvanising).
Functional: nickel plating for wear resistance.
SETUP for silver plating a copper spoon:
ELECTROLYTE: silver nitrate solution (AgNO₃).
ANODE: silver (dissolves to replenish Ag⁺ ions in solution).
CATHODE: the copper spoon (coated with silver).
AT CATHODE:
Ag⁺ + e⁻ → Ag (silver deposits on the spoon)
AT ANODE:
Ag → Ag⁺ + e⁻ (silver anode dissolves, maintaining Ag⁺ concentration)
The anode mass decreases and the cathode mass increases by the same amount.
⚠️ Common Mistake
In copper purification, the IMPURE copper is the ANODE (it dissolves). The PURE copper builds up on the CATHODE. Students often get these the wrong way round. The anions in the electrolyte (SO₄²⁻) do not move to either electrode in copper purification — only copper ions are discharged.